An assembly line essentially consists of a plurality of assembly units, for example assembly cells. In the prior art a conveyor belt transports workpiece carriers from assembly cell to assembly cell. Workpieces are held in the workpiece carrier. The workpieces are processed in each assembly cell. An assembly cell usually contains at least one processing unit. One to four, typically two, manufacturing steps are usually carried out in one assembly cell. Also within the assembly cell the conveyor belt transports the workpiece carrier with the workpieces from processing unit to processing unit.
The conveyor belt is usually arranged to be continuous and moves uniformly. Naturally, in order to carry out a manufacturing step in a processing unit, the workpiece carrier must be prevented from continuing to move on the conveyor belt. For this, two steps are usually provided: electrically- or pneumatically-operated stoppers are mounted on an input side of the processing unit. The workpiece carrier is either removed from the conveyor belt by means of this stopper, or alternatively it is mechanically blocked so that the previous adhesion between the workpiece carrier and the conveyor belt is removed and the conveyor belt continues to move under the workpiece carriers while these are stopped.
The stoppers ensure that incoming workpiece carriers stop in front of the processing unit. Only one workpiece carrier reaches the processing unit at any one time. The other workpiece carriers wait at a distance from the processing unit until the current manufacturing step is completed. During the entire time the conveyor belt continues to run at constant speed. The workpiece carrier is now transferred from the input side into a processing position in the processing unit. In this connection, the conveyor belt is either used again or provision can also be made for a gripper arm or similar to place the workpiece carrier in the processing position. In order to hold the workpiece carrier in the processing position, special mechanical elements are provided, such as locating pins for example, which likewise have to be electrically or pneumatically actuated.
Precise processing of the workpiece can be carried out in the processing position. However, precise control of the position of the workpiece carrier is not possible in the prior art. Until now, the tool is equipped with a servo-driven motion axis to facilitate movements in the transportation direction of the conveyor belt.
The use of electrically- or pneumatically-operated stoppers, as well as elements for locating the workpiece carrier in the processing unit, involves relatively high expenditure. Electrically- or pneumatically-operated components are susceptible to faults. Also, their faults can only be simulated and analyzed with difficulty via software. If a fault occurs, then this usually has to be laboriously located by the maintenance personnel.
The system is relatively inflexible, which makes it difficult to reset individual assembly cells. In particular, this is then a disadvantage if a basic assembly line set-up is used and different products are to be manufactured in not particularly large batch quantities at any one time, so that frequent modifications are necessary.